When the body of a cabin or the like is placed and fixed on the frame of an automobile, vibration-preventing devices are interposed between the frame and the body so as to absorb and dampen vibrations. Thus, vibrations transmitted from tires to the frame during operation of the automobile may be prevented from propagating to the body.
In general, vibrations originating during the operation of vehicles have high and medium frequencies. Therefore, vibration-preventing rubber members having comparatively small spring constants are usually used as vibration-preventing devices. Such vibration-preventing devices, however, do not satisfactorily dampen vibrations in a low-frequency region such as, for example, vibrations attributable to undulations of the surface of a bad road.
Multiple mounting type vibration-preventing rubber devices have been developed which do not only absorb high- and medium frequency vibrations but also dampen low-frequency vibrations.
As shown in FIG. 6, a multiple mounting vibration-preventing rubber device 1 includes, for example, an upper plate 2, connected to a lower plate 4, by an inner cylinder 3. A partition wall 6, having a hub 5, fitted outside the inner cylinder 3, is interposed between the upper plate 2 and the lower plate 4.
An upper member of a vibration-preventing rubber member 7, in the shape of a ring, is interposed between the partition wall 6 and the upper plate 2. An upper elastic film 8 is sealingly fixed inside the upper vibration-preventing rubber member 7 such that the lower end thereof is spliced to the hub 5 of the partition wall 6. The upper outer end of the upper elastic film 8 is sandwiched between the upper end of the upper vibrationpreventing rubber member 7 and the upper plate 2. The outer end of the upper plate 2 is caulked. This structure defines an upper damper liquid chamber 9.
A lower damper liquid chamber 12 is defined as follows. A lower vibration-preventing rubber member 10, in the shape of a ring, is interposed between the partition wall 6 and the lower plate 4. A lower elastic film 11 is sealingly fixed inside the lower vibration-preventing rubber member 10. The upper end of the lower vibration-preventing rubber member 10 is spliced to the hub 5 of the partition wall 6, and the lower outer end thereof is sandwiched between the lower end of the lower vibration-preventing rubber member 10 and the lower plate 4. The outer end of the lower plate 4 is caulked.
The upper damper liquid chamber 9 and the lower damper liquid chamber 12 communicate through an orifice 13 provided in the partition wall 6. A damper liquid, which is a noncompressible fluid such as coolant, is tightly sealed in the chambers.
Outer peripheral parts of the partition wall 6 of the vibration-preventing device 1 are clamped to the frame 14 of a vehicle by bolts and nuts. The body 15 of, for example, a cabin placed on the upper plate 2, is fixed by a bolt 16, inserted through the inner cylinder 3, and a nut 17. High-frequency and medium-frequency vibrations arising in the frame 14 during operation of the vehicle are absorbed by the vibration-preventing members 7 and 10, while low-frequency vibrations are dampened by flow of the damper liquid through the orifice 13 communicating the damper liquid chambers 9 and 12. Thus, the vibrations from the relative up and down motion of the partition wall 6 and the inner cylinder 3 are attenuated under the damping action of the orifice 13.
The relationship between a frequency corresponding to a peak damping coefficient, namely, the resonance frequency f.sub.n of the damper liquid within the orifice 13, the volume modulus k.sub.1 of the upper damper liquid chamber g, the volume modulus k.sub.2 of the lower damper liquid chamber 12, the aperture area S of the orifice 13, the length l of the orifice 13, and the specific gravity .rho. of the damper liquid, is as follows: EQU f.sub.n .alpha..sqroot.S(k.sub.1 +K.sub.2 /.rho.l
That is, the resonance frequency f.sub.n of the damper liquid may be lowered by reducing the aperture area S of the orifice 13, lowering the volume moduli k.sub.1 and k.sub.2 of the respective damper liquid chambers 9 and 12, or increasing the length of the orifice 13.
In the above vibration-preventing rubber device, when the resonance frequency f.sub.n of the damper liquid within the orifice 13 is lowered to attain satisfactory vibration damping of the components of the relative vertical vibrations of the vehicular frame 14 and the body 15 in the low-frequency region, the aperture area S of the orifice 13 must be reduced. However, when the aperture area S is made smaller than a predetermined value, flow resistance increases thus disadvantageously decreasing the peak value of the damping coefficient which degrades the vibration damping function.
FIG. 3 is a graph showing the correlation between the damping coefficient R of the relative vertical vibrations, shown on the ordinate axis, and the frequency [Hz], shown on the abscissa, with the aperture area S of the orifice 13 being a parameter. The order of values S.sub.1, S.sub.2 and S.sub.3 represent increasing values for the aperture area S. Thus, to enhance the vibration damping function of the vibration-preventing device 1 with a large peak value of the damping coefficient and a low resonance frequency f.sub.n, the length l of the orifice 13 needs to be increased while the minimum limit value of the aperture area S thereof remains constant. Since, however, the orifice 13 is a straight pipe, the length of the orifice is limited by the dimensions of the vibration-preventing rubber device 1, so it is difficult to attain a desired damping vibration function.
FIG. 4 is a graph showing the correlation between the damping coefficient R of the relative vertical vibrations, shown on the ordinate axis, and the frequency [Hz], shown on the abscissa, with the length l of the orifice 13 being a parameter. The order l.sub.1, l.sub.2 and l.sub.3 represents increasing lengths l.